1. Field of the Invention
The present invention is directed to the production of stable aqueous polyurethane-urea dispersions which contain epoxide resins that are not dispersible in water and the use of these dispersions for the production of coatings having improved humidity resistance.
2. Description of the Prior Art
The production of linear or cross-linked, aqueous, polyurethane-urea dispersions is known as shown by U.S. Pat. Nos. 3,479,310; 4,066,591: 4,092,286: 4,108,814; 4,237,264; and 4,238,378 which disclose linear polyurethane-ureas and U.S. Pat. Nos. 3,870,684; 4,203,883: and 4,408,008 which disclose cross-linked polyurethane-ureas. The aqueous polyurethane-urea dispersions may be used for a wide range of commercial applications such as adhesives or coatings for various substrates including textile fabrics, plastic, wood, glass fibers and metals. Chemical resistance, abrasion resistance, toughness, tensile strength, elasticity and durability are among the many desirable properties of these coatings. In some cases these properties of coatings prepared from aqueous polyurethane-urea dispersions have equaled or even surpassed the performance levels of coatings obtained from solvent-based polyurethane lacquers. However, one property of coatings prepared from aqueous polyurethane-urea dispersions which needs to be improved in order to match the performance of coatings prepared from solvent-based polyurethane lacquers is the humidity resistance. This property is adversely affected by the hydrophilic materials necessary for insuring the dispersibility of the polyurethane-ureas in an aqueous medium.
Accordingly, it is an object of the present invention to provide aqueous polyurethane-urea dispersions which may be used to prepare coatings having improved humidity resistance. It is an additional object of the present invention to provide aqueous polyurethane-urea dispersions which may be used to prepare coatings having excellent adhesion, chemical resistance, abrasion resistance, toughness, tensile strength, elasticity and durability.
Surprisingly, it has been found that these objectives may be achieved according to the present invention by mixing a water-dispersible isocyanate-terminated prepolymer with an epoxy resin which is not dispersible in water prior to dispersing the prepolymer in water and subsequently chain extending the prepolymer.
There have been many previous attempts to improve the hydrolytic stability or water resistance of polyurethane-ureas by the incorporation of epoxy resins. For example, U.S. Pat. Nos. 4,160,065 and 4,190,567 are directed to the production of cationic polyurethane-ureas with terminal amine groups which are subsequently reacted with epoxy resins in order to improve the water resistance of the resulting films or coatings. Conversely, U.S. Pat. Nos. 3,931,116 and 3,971,745 are directed to the production of anionic polyurethane-ureas having terminal amino groups which are subsequently chain extended with epoxy resins to improve the water resistance. U.S. Pat. No. 4,036,906 is directed to blends of polyurethane prepolymers, with epoxy resins which are subsequently chain extended with polyamines in order to improve the water resistance of the resulting coatings. Finally, U.S. Pat. No. 4,403,085 is directed to blends of blocked isocyanate-terminated prepolymers and epoxy resins which are subsequently cured to form films having improved properties.
In most of the preceding references, it is stated that the water resistance of the resulting films and coatings is improved. In these instances "water resistance" refers to immersing the films or coatings in water for a period of at least several hours and then retesting the coatings to determine if there has been any reduction in the tensile properties of the coatings or films because of their immersion in water. It is known that the polyesters which are predominantly used to prepare the polyurethane-ureas or the amine-terminated polyurethane-ureas are susceptible to attack by water. It is also known that epoxy resins may be chain extended with amines in the same manner as isocyanate-terminated prepolymers. Accordingly, the improvement shown by these references may be explained by the fact that a portion of the isocyanate-terminated prepolymer is replaced by an epoxy resin and subsequently both of these components are chain extended with an amine. The result is that the final product contains less of the polyurethane-ureas and, accordingly, less of the degradable polyester resins, resulting in improved water resistance.
To the contrary, the present invention is directed to a mixture of stable, aqueous polyurethane-urea dispersions with epoxy resins which are not water dispersible. The epoxy resins are not reacted with the aqueous polyurethane-urea dispersions until the coating is cured on the substrate at elevated temperatures. At this time, the epoxide groups react with the acid groups which are the precursors of the anionic hydrophilc groups which provide hydrophilicity for dispersing the aqueous polyurethane-urea dispersions.
U.S. Pat. No. 4,306,998 is directed to a process for preparing aqueous polyurethane-urea dispersions containing additives which are not soluble or dispersible in water by incorporating these additives with the water-dispersible, isocyanate-terminated prepolymer precursors for the polyurethane-urea dispersions. Evan though this reference includes epoxy resins among an extensive list of additives which may be incorporated, this reference does not recognize the improvements in humidity resistance which may be obtained incorporating epoxy resins in accordance with the present invention.
Finally, Bulletin 345 from Witco Chemical Corporation concerning Witcobond W-290H urethane latex suggests the incorporation of a water dispersible epoxy for the purpose of improving the water resistance of the resulting coatings or films after 24 hours immersion in water.